Optimism and Arbitrum are Layer 2 rollup scaling solutions for Ethereum. Put simply, Layer 2's are networks built on top of mainnet Ethereum (also known as Layer 1) designed to help scale mainnet and make using it more efficient. On layer 2, transactions are over 100x less expensive and occur over 100x faster than they do on Ethereum mainnet. Both of these networks are also fully EVM compatible, meaning smart contracts on the networks are written exactly the same way as they would be on mainnet and they all share the same tooling ecosystem (ENS, Etherscan, Uniswap, Metamask, etc).
For further information on Layer 2, see here.